Conventional gas turbine engines, such as those used in aircraft, are enclosed in an engine casing and include a plurality of turbine blades secured to a drum. Such gas turbine engines, typically mounted on the wing of an aircraft, are frequently damaged by foreign objects, such as sand particles, stones, or other objects ingested by the engine during takeoff. These foreign objects ingested by the air turbine engine often cause generally V-shaped nicks or chips on impact along the leading edge of the affected turbine blades. The process of replacing a turbine blade is very expensive, so repair in place is desirable when compared to replacement.
In order to prevent such notches or nicks from becoming more pronounced and potentially cracking the turbine blade, it is desirable to detect the nicks or notches early and, if possible, repair or blend the defects in the turbine blades. In general the term blending is used in the art for the process of smoothing a V-shaped notch or nick into a more U-shaped configuration.
The detection process involves a visual inspection of each turbine blade through a borescope or endoscope passed through observation ports or holes in the engine casing. The borescope, a fiber optic cable connected to a light source, is inserted through borescope openings within the engine case and into the engine. The small borescope openings are disposed throughout the engine case. If a turbine blade having excessive damage is observed, the engine must be removed from the wing of the aircraft, and then disassembled to expose the damaged blade. Only then can the blade be accessed and repaired or replaced. This procedure is time-consuming and extremely expensive. Consequently, more practical techniques for repairing or blending notches or defects on an aircraft turbine blade have been developed. For purposes of the present document, the word “endoscope” may be interpreted to include rigid borescopes, flexible fiber optic borescopes and videoscopes or any similar device.
One type of apparatus used to blend defects on turbine blades in the manner described above uses a rotary grinding head or tool located at the end of a blending tool. The tool may be passed through the observation ports in the engine casing. U.S. Pat. Nos. 5,644,394; 5,803,680 and 5,475,485 disclose such apparatus. One difficulty with tools using rotary heads is that the rotational speed required to blend the defect is so high that the surface of the turbine blade becomes very hot due to friction. Because most turbine blades are made of titanium, the integrity of the titanium may be compromised at high temperatures. The titanium metal may actually melt or deform at high enough temperatures.
An alternative to a tool which rotates a grinding head is disclosed in U.S. Pat. No. 5,102,221. This patent discloses an apparatus for repairing or blending defects on a turbine blade using a reciprocating motion, as opposed to a rotary motion. Again, this apparatus is used with an endoscope. The apparatus disclosed in this patent is difficult to use and subject to failure due to the configuration and operation of the apparatus. Therefore, there is a need for a grinding apparatus to blend defects on turbine blades which is user-friendly and utilizes a reciprocating motion, as opposed to a rotary motion.